1. Field of the Invention
The present invention relates to the transfer of workpieces such as semiconductor wafers from a storage and transport pod to a process tool, and in particular to a system for ensuring a properly aligned position of a pod door within the opening in a pod shell upon location of the pod at a tool load port.
2. Description of Related Art
A SMIF system proposed by the Hewlett-Packard Company is disclosed in U.S. Pat. Nos. 4,532,970 and 4,534,389. The purpose of a SMIF system is to reduce particle fluxes onto semiconductor wafers during storage and transport of the wafers through the semiconductor fabrication process. This purpose is accomplished, in part, by mechanically ensuring that during storage and transport, the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers, and by ensuring that particles from the ambient environment do not enter the immediate wafer environment.
A SMIF system has three main components: (1) minimum volume, sealed pods used for storing and transporting wafers and/or wafer cassettes; (2) an input/output (I/O) minienvironment located on a semiconductor processing tool to provide a miniature clean space (upon being filled with clean air) in which exposed wafers and/or wafer cassettes may be transferred to and from the interior of the processing tool; and (3) an interface for transferring the wafers and/or wafer cassettes between the SMIF pods and the SMIF minienvironment without exposure of the wafers or cassettes to particulates. Further details of one proposed SMIF system are described in the paper entitled xe2x80x9cSMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING,xe2x80x9d by Mihir Parikh and Ulhrich Kaempf, Solid State Technology, July 1984, pp. 111-115.
Systems of the above type are concerned with particle sizes which range from below 0.02 microns (xcexcm) to above 200 xcexcm. Particles with these sizes can be very damaging in semiconductor processing because of the small geometries employed in fabricating semiconductor devices. Typical advanced semiconductor processes today employ geometries which are one-half xcexcm and under. Unwanted contamination particles which have geometries measuring greater than 0.1 xcexcm substantially interfere with 1 xcexcm geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries which today in research and development labs approach 0.1 xcexcm and below. In the future, geometries will become smaller and smaller and hence smaller and smaller contamination particles and molecular contaminants become of interest.
SMIF pods are in general comprised of a pod door which mates with a pod shell to provide a sealed environment in which wafers may be stored and transferred. So called xe2x80x9cbottom openingxe2x80x9d pods are known, where the pod door is horizontally provided at the bottom of the pod, and the wafers are supported in a cassette which is in turn supported on the pod door. It is also known to provide xe2x80x9cfront openingxe2x80x9d pods, in which the pod door is located in a vertical plane, and the wafers are supported either in a cassette mounted within the pod shell, or to shelves mounted directly in the pod shell itself.
In order to transfer wafers between a SMIF pod and a process tool within a wafer fab, a pod is typically loaded either manually or automatedly onto a load port on a front of the tool so that the pod door lies adjacent the port door of the process tool. Thereafter, mechanisms within the load port advance the pod to the port, where the port door decouples the pod door from the pod shell and moves the pod door and port door together into the minienvironment and then off to the side. The pod shell remains in position against the interface port to maintain a seal at the port and to define a sealed, clean environment including the interior of the process tool and pod shell. A wafer handling robot within the process tool may thereafter access particular wafers supported in the pod shell for transfer between the pod and the process tool.
During wafer storage and transport, the pod door is typically held affixed to the pod shell by a latch assembly such as disclosed in U.S. Pat. No. 4,995,430, entitled xe2x80x9cSealable Transportable Container Having Improved Latch Mechanismxe2x80x9d, to Bonora et al., which patent is owned by the assignee of the present application. The mechanism disclosed therein includes a two-stage latching operation to securely latch a pod door to a pod shell as shown in prior art FIGS. 1 and 2A-2B. The latch assembly is mounted within the pod door, and includes a latch hub 10 which engages first and second translating latch plates 12. Mechanisms in the form of driven latch keys extend from the port door into slots 13 formed in the latch hub to thereby rotate the latch hubs clockwise and counterclockwise. Rotation of each latch hub 10 will cause translation of the first and second latch plates 12 in opposite directions.
FIG. 1 is a front view of an interior of the pod door illustrating the latch assembly in the first stage of the door latching operation. When a pod door is returned from its engagement with the port door to the pod, mechanisms within the port door rotate the latch hub 10 to thereby translate the latch plates 12 outwardly so that latch fingers 14 on the ends of the latch plates 12 extend in the direction of arrows A into grooves formed in the pod shell. FIG. 2A is a side view through line 2xe2x80x942 of the latch assembly shown in FIG. 1, and FIG. 2B is a side view as in FIG. 2A but illustrating the second stage of the door latching operation. In particular, the latch hub 10 further includes a pair of ramps 16 so that, after the fingers have engaged within the grooves of the pod shell, further rotation of the hub causes the ends 18 of the latch plates engaged with the hub to ride up the ramps. This causes the latch plates to pivot in the direction of arrows B, about axes lying in the plane of each latch plate and perpendicular to the direction of latch plate translation. The effect of this pivoting during the second stage is to pull the pod door tightly against the pod shell to thereby provide a firm, airtight seal between the pod door and shell.
In order to separate a pod door from a pod shell, as when a pod is initially loaded onto a load port interface for wafer transfer, mechanisms within the port door engage the rotatable hub 10 and rotate the hub in the opposite direction than for pod latching. This rotation disengages the latch fingers 14 from the pod shell and allows separation of the pod door from the pod shell.
In order for the decoupling mechanisms within the port door to properly engage and decouple the pod door from the pod shell, it is important that the pod door be properly positioned on the load port. Additionally, port doors include guide pins which register within slots in the pod door with relatively little tolerances. Moreover, in front opening pods, the pods are supported on their bottom surface, but must be accurately registered against a vertical surface (i.e., the port door). All of these factors require that the pod door be properly positioned within the pod when the pod is loaded onto the load port.
However, in order to ensure easy location of the pod door in the pod shell opening, and to prevent frictional engagement between the pod door and pod shell when the door is returned to the pod, a clearance is left on all sides between the pod door and the pod shell. While the clearance between the pod door and pod shell is important when sealing the pod, this clearance may also result in the pod door being off-center when attached to the pod shell or thereafter. This off-centering may occur one of several ways. For example, in front opening pods, the weight of the pod door may cause the door to sag downward in the pod shell opening. Additionally, in either front or bottom opening pods, an unexpected shock or jolt to the pod during transport may cause the pod door to shift off-center within the pod shell opening. As indicated above, unless the pod door is properly centered with respect to the pod shell, the port door gripping mechanisms may not be able to properly engage the pod door latch mechanism, and/or the pod door may not properly align over the registration pins on the port. Further still, improper alignment may result in an undesirable frictional contact between the latch driving mechanisms in the port door and the pod door latch assembly, which frictional contact may generate harmful particulates.
It is therefore an advantage of the present invention to provide a system for ensuring that a pod door is properly positioned for engagement by a port door upon loading of a pod onto a load port interface.
It is a further advantage of the present invention to provide a mechanism for preventing a pod door in a front opening pod from sagging downward in the pod shell opening.
It is another advantage of the present invention to provide a device for preventing a pod door from shifting out of proper position in the pod shell opening in the event the pod receives an unexpected jolt or shock.
It is a still further advantage of the present invention to provide a system for maintaining a pod door centered with respect to a pod shell, which system may be easily incorporated into an existing pod design.
These and other advantages are provided by the present invention which in general relates to a system for ensuring that a pod door is properly positioned within a pod shell during storage, transfer and loading of the pod onto a load port. A preferred embodiment of the present invention may be added to a conventional door latching assembly within a pod door, and may be driven by the same mechanisms in the port door that actuate the door latching assembly. The door positioning assembly according to the present invention includes a cam affixed to each of the rotating latch hubs of a pod door latching assembly. The cams are located on a side of the latch hubs opposite that including the ramps used to pivot the pod door into tight engagement with the pod cover. The door positioning assembly further includes a cam follower mounted around each of the cams, which followers include arm portions that extend out toward an edge of the pod door.
When the pod door and shell are separated, the arm portions are held in a retracted position completely contained within the footprint of the pod door. However, upon rejoining the pod door to the pod shell, mechanisms in the port door rotate the latch hub cam. Cam rotation causes translation of the arm portions of the cam followers so that the ends of the arm portions extend out beyond an edge of the pod door and against a surface of the pod shell. In their extended positions, the arm portions maintain a desired positioning of the pod door within the pod shell opening. Moreover, as the arm portions remain in their extended positions after the pod door is coupled to the pod, the positioning assemblies will ensure that the pod door is properly positioned in the pod shell opening during pod transfer and loading onto a load port.
A preferred embodiment of the present invention includes two arm portions capable of extending out from a bottom edge of a pod door of a 300 mm front opening pod to prevent sagging of the pod door in the pod shell opening. However, alternative embodiments of the present invention may include a door positioning assembly having a plurality of arm portions extending outward from two or more sides of the pod door. Such an embodiment could be used for bottom or side opening pods.